In general, apparatuses using power such as machine tools or vehicles do not use power transmitted from a power source as it is, but change a speed and torque of inputted power by using a transmission so as to obtain a necessary speed and torque.
The transmission increases or decreases the number of revolution of an input shaft at an output shaft by coupling gears having different diameters and gear ratios to each other to induce a gear shift.
However, since the gear shift is realized by the coupling the gears to each other in the transmission according to the related art, noises and abrasion due to friction may occur. Particularly, the transmission has a large size and occupies a large space to deteriorate efficiency of power transmission.